Porous carbon substrates used to constitute a gas diffusion material of fuel cells, respectively obtained by binding carbon fibers by means of a carbonized resin, are described in JP 06-20710 A, JP 07-326362 A and JP 07-220735 A. However, these publicly known porous carbon substrates have such problems that they are inconvenient to handle and that it is difficult to provide them in long continuous forms.
Any of these publicly known porous carbon substrates is produced by a process comprising a paper making step of making carbon fiber paper, a resin impregnation step of impregnating the carbon fiber paper with a thermosetting resin, a compression step of compressing the resin-impregnated carbon fiber paper for forming it, and a carbonization step of burning the compressed resin-impregnated carbon fiber paper provided as a fiber sheet precursor.
The sheet processed in the paper making step and in the resin impregnation step has a continuous and long form. However, the sheet is cut at a predetermined length, before it is supplied to the compression step. The cut sheets, namely, unit sheets are fed to the compression step one by one and each of the unit sheets fed one by one is pressed by a batch type flat plate press. In the subsequent carbonization step, a batch type heating furnace is used for carbonizing the resin. The compression step and the carbonization step are batch processes, since the obtained porous carbon substrate is inconvenient to handle. That is, the reason is that it was difficult to produce a long continuous porous carbon substrate. Therefore, the reason is that a porous carbon fiber substrate wound as a roll could not be produced.
In general, a batch process is low in productivity compared with a continuous process. Also in the above-mentioned carbonization step, the heating rate that can be employed in a batch type heating furnace is up to several degrees centigrade per minute at the highest. Therefore, there are such problems that the productivity of the porous carbon substrate is low and that the production cost is high.
On the other hand, WO 01/56103 A1 discloses an electrode substrate capable of being wound as a roll and a production process thereof. This document proposes to use carbon fibers with a small fiber diameter for enhancing, for example, the bending strength of the sheet. However, if the fiber diameter of carbon fibers is made smaller, the bending modulus of elasticity of the obtained porous carbon substrate tends to be high. That is, there is a problem that a larger force is necessary for winding the substrate as a roll. Furthermore, the porous carbon substrate obtained by using carbon fibers having a small fiber diameter is small in the size of the pores formed in the substrate. Therefore, in the case where the substrate is used as a gas diffusion material, there arises such a problem that, for example, gas diffusibility and water draining capacity decline.
Moreover, the document proposes to use a continuous hot press with a pair of endless belts or a continuous roll press, for allowing continuous operation in the compression step. However, for compression forming of carbon fiber paper containing a thermosetting resin, the state of heating and pressing must be kept for a period of time enough to allow the curing reaction of the resin to progress to some extent, usually for several minutes to tens of minutes. Therefore, an apparatus for applying a pressure to a sheet as a linear pressure like the belt press or roll press has a problem that it is difficult to compress a fiber sheet precursor for forming it at a thickness accuracy equivalent to that of a batch type flat plate press.
Still furthermore, the document proposes to burn the sheet in a continuous burning furnace, for allowing continuous operation in the carbonization step. Certainly, continuous burning can remarkably enhance the productivity compared with that of batch type. However, if the productivity is enhanced, the thermosetting resin contained in the fiber sheet precursor is suddenly carbonized. Therefore, if a continuous burning furnace is merely used, there arise such problems that the separation at the interfaces where the carbonized resin and the carbon fibers are bound to each other occurs considerably and that the carbonized resin is remarkably cracked, respectively due to the sudden carbonization shrinkage of the resin.